The use of metal sheathed heaters is well known in the prior art. Typically, these heaters use resistance heating wherein a resistance heating wire or heater cable is encased in a metal sheath. The metal sheath is in contact with the item or material to be heated. These heaters are often referred to as belly-band, crankcase, compressor or sump heaters and are often times used to heat refrigeration compressors or air-conditioning compressors. The heater can employ a standard hose clamp or other type of clamping arrangement for attachment to the compressor. The standard hose clamp is cut in two pieces with each piece affixed (welded for example) to opposite ends of the heater's metal sheath. Assembly of the heater to the compressor is accomplished by engaging the two ends of the clamp as intended and then tightening the assembly around the selected compressor location. This type of heater construction can also be used for heating containers such as barrels, heating pipes, etc.
The belly-band heater has an insulated electric lead wire exiting each end of the metal sheath. A frequent requirement in the use of these heaters is for the lead wires to be routed in standard metal conduit. Further, it is often required that the conduit enclose the lead wires from the point where each lead exits the heater sheath to where the lead wires enter an electrical junction box or boxes.
FIG. 1 shows a typical metal sheathed heater or electric belly-band heater designated by the reference numeral 10 and including hose clamp pieces 1 and 3, and a screw mechanism 5. A metal sheath 7 extends between the two pieces 1 and 3, with the hose clamp pieces attached to the sheath by welding or the like. The metal sheath 7 encases an electrically insulated resistance heating wire or heater cable 9 and includes a fluted strip portion 8, which interfaces with the equipment or material requiring heating.
In these types of metal sheathed heaters, it is well known in the industry that the heater cable is composed of resistance wire spiraled around a flexible core made of an electrically insulated and thermally resistant material such as fiberglass or other suitable material. This element is commonly referred to as a “heater core wire”. After the heater core wire is uniformly coated with an insulating material having sufficient mechanical and electrical resistance properties so as to remain flexible yet electrically isolated, it is normally called a “heater cable”. The insulating material is often silicone or a thermosetting plastic with adequate thermal properties for its intended use.
In connecting the heater cable to the lead wires, a small length of insulation is stripped from each end of the heater cable. Two flexible electrically insulated stranded lead wires with a small length of insulation stripped from one end of each wire are electrically connected, one to each end of the heater cable, by crimping or splicing the stripped ends of the heater cable to stripped ends of the lead wires. The connector used is a properly selected metal splice connector with sufficient temperature resistance, corrosion resistance, mechanical strength and formability to make a secure electrical bond. In the embodiment shown in FIG. 1, the lead wires 11 are connected to the heater cable using connections that are in turn encased in the metal sheath as is disclosed in co-pending patent application Ser. No. 11/064,845 filed, owned by the assignee of this application. The connection between the lead wires and heater cable can be made outside the metal sheath if so desired.
These types of heaters are commercially available from Tutco, Inc, of Cookeville, Tenn., one being Model CH, and are also disclosed in U.S. Pat. No. 6,844,531 to Kirby, which is herein incorporated in its entirety by reference.
In the prior art, these types of metal sheathed heaters e.g., electric resistance compressor heaters, when installed on a compressor that is part of a total system or controlling unit, remain constantly powered regardless of temperature as long as the controlling unit that the heater serves demands power. During certain periods, temperature conditions occur for which the electric resistance compressor heater does not need to operate even though the controlling unit demands power. As a result electrical energy is consumed which is a waste of resources. The use of positive temperature coefficient resistance heaters for heating compressors only partially reduces the use of electrical energy and is not a solution to this problem.
Attempts to use various thermostats to control electric resistance compressor heaters have not been successful. Compressor heaters must operate in moist to wet environments. Therefore, finding a practical solution to this dilemma remains a problem for those seeking to use metal sheathed heaters in an efficient manner.
The present invention overcomes the problems noted above by providing a metal sheathed heater and a thermostat assembly, with the thermostat interposed between the heater and a power source using a mechanically strong, abrasive resistant, moisture resistant, electrically insulating joint.